The Athabasca oil sands in north east part of the province of Alberta now comprise one of the largest remaining oil reserves in the world. The oil bearing geological formation is shallow near the Alberta-Saskatchewan border and is located a few tens of meters below the surface there. The formation slopes to the west such that, in 10-20 km, it is more than 300 m below the surface. As a result, there are two methods being used to extract the oil from the ground. Mining is economically viable where the formation is less than 70 m below the surface. “In Situ” processes, where horizontal oil wells are drilled, are used when the formation is deeper than is economically minable. Mining has a higher recovery ratio since the entire ore body is accessible.
Oil sand is comprised of three main components: solids, oil and water. The solids can be sub-classed into sands and silt/clay particles. The sand particles are larger than the silt/clay particles, ranging from stones and pebbles down to fines near 1 μm diameter. Silts and clays are finer particles and tend to be between 100 and 0.1 μm in size. FIG. 1 shows the solids particle size distribution for a number of types of ore bodies in the Athabasca region. Curves 1, 2 and 3 correspond to ore bodies with higher sand content. Curves 4 and 5 correspond to ore body with higher silt and clay content since more than 60 wt % of the particles are smaller than 100 μm. There are ore bodies with even higher silt/clay content. As a broad rule, the bitumen content of the ore varies inversely with the concentration of fines. The term “fines” is applied to particles smaller than 44 μm. Vertical line A represents this boundary on FIG. 1. Generally, the wt % passing of fines is less than 30% for ore bodies bearing more than 7% bitumen. 7% bitumen is considered near the lower limit of economic recoverability for current technology. Curves 4 and 5 would not be representative of economically viable ore bodies since the fines contents are 38% and 43% respectively.
The oil can be sub-classed into bitumen and asphaltenes. Bitumen is the desired component because it can be readily upgraded into synthetic crude oil. Asphaltenes are components with high carbon content. They also contain sulfur, metals and aromatic molecule groups. The molecules are large but also form solid crystalline conglomerations of molecules. The asphaltene particles range from nanometer to micrometer scales and they form 17-18 wt % of the oil mixture. While they are solid they are considered to be part of the oil component. They are undesirable because they increase the viscosity of the bitumen mixture and, more importantly, because they disrupt the upgrading process. Asphaltenes tend to “coke” the upgrading vessels and equipment, requiring expensive maintenance and down time.
The water component of the oil sand is important. Solid particles are hydrophilic and tend to be within the water component of the ore. The sand particles are “wetted” with a fine layer of water, approximately 10 nm in thickness. Where the sand grains touch, a pendular ring of water forms by surface tension. Oil is hydrophobic and coats the sand grains outside of the water layer. This is an important factor. It means that when the ore is immersed in solvents, the bitumen is accessible to the solvent without disturbing the water layer. The remainder of the interstitial space between the oil coated sand grains is filled with water. This interstitial water component contains a large part of the fines content of the oil sand because the fines are hydrophilic. Nevertheless, there are still fines incorporated into oil phase of the ore and these become the most difficult solids to separate from the bitumen.
The mass fraction ranges for the various components in commercially viable oil sand are listed in Table 1.
TABLE 1Oil Sand Ore Component Mass FractionsComponentWt %sand81-88silt/claywater3-6bitumen 5-13asphaltenes1-3
It is important to emphasize that the separation of the solids from the bitumen must be nearly complete for commercial processing. For a low grade ore of 7% bitumen, 99.94% of the solids must be removed from the bitumen stream to meet the pipeline specification of less than 0.5 wt % solids and water.
Bitumen and Solvent Properties
Bitumen is highly viscous. To separate the bitumen from the solids, the viscosity of the bitumen must be reduced so that it will flow away from the sand. The two main methods of reducing the bitumen viscosity involve increasing the bitumen temperature, or diluting the bitumen with a solvent.
FIG. 2 shows an exponential decrease in viscosity with temperature. The viscosity of pure bitumen (0% solvent) drops three orders of magnitude with a temperature increase of 60° C. For this reason, all of the processes used in the current art involve some amount of heating. However, this viscosity decrease is insufficient to allow the solids to be separated from the bitumen. At a temperature of 80° C., pure bitumen has a viscosity of 1000 mPa-s, which is similar to that of thick syrup. Further viscosity reduction is required, and is accomplished by means of solvents. FIG. 2 shows that propane solvent, mixed in a 50% mass fraction reduces the viscosity of the bitumen by more than six orders of magnitude. Therefore, the addition of propane solvent offers a substantial advantage over any known heating methods.
There are two broad classes of solvents: Paraffinic solvents and aromatic solvents. Paraffinic solvents are non-ring hydrocarbon chain molecules ranging from propane, with three carbon atoms, to very long chains. Methane and ethane (one and two carbon atoms) have triple points lower than atmospheric temperature and pressure and therefore cannot be liquids at the temperatures and pressures normally encountered in these applications. Therefore, methane and ethane cannot be used as solvents. Shorter chain paraffins make better solvents because of their lower density and viscosity and also because they do not dissolve asphaltenes well. The disadvantage of these solvents is that they are either vapors at atmospheric pressure and 20-80° C., or they have very high vapor pressures and thus tend to evaporate even if they are liquids.
Table 2 shows that the boiling temperature of the solvents increases with higher molecular weight. Propane, butane, pentane or hexane cannot be used as solvents at atmospheric pressure and 80° C. because they would all boil away. Heptane would experience significant losses to the atmosphere because it would evaporate quickly.
TABLE 2Solvent PropertiesSolvent Properties (1)MolecularWeightBoilingVaporDynamickg/kg-Temp. (2)PressureDensityViscositySolventmole° C.MPa (abs)kg/m3mPa-sPropane44.0956−42.123.2643650.047Butane58.1222−0.501.0624970.093Pentane72.148836.060.3925590.137Hexane86.175468.720.1535990.175Heptane100.201998.380.0616290.223Naphtha(3)30-200(3)600-8500.980(1) At 82.2° C. and at the vapor pressure noted.(2) At atmospheric pressure, 101.325 kPa.(3) Naphtha is a mixture of solvents, containing paraffins from C5-C12 as well as aromatic solvents. All values in this row are approximate and depend on the particular grade of naphtha.
As the molecular weight of a paraffinic solvent increases, its behavior becomes more like aromatic solvents in its ability to dissolve asphaltenes.
Aromatic solvents have heavier molecular weights and are liquids at atmospheric pressure and temperature. Naphtha, a mixture of paraffinic and aromatic elements, is a commonly used solvent.
Clark Hot Water Extraction Process
The current state of the art is the Clark hot water extraction process. This process is described in a number of Canadian patents by Karl Adolf Clark, including 289,058 and 448,231. Numerous improvement patents for this process also exist. The Clark process requires substantial physical equipment. Ore is mined, crushed and mixed with hot water and caustic in large atmospheric pump boxes. The slurry mixture is pumped from the pump boxes in large hydrotransport pipelines to an extraction plant. The hydrotransport pipeline serves two purposes; to transport the ore and to ensure that the oil, water and sand are thoroughly mixed. When the slurry reaches the extraction plant it flows into large primary separation cells where, with recirculation pumps, an oily froth is created and most of the sand is separated from the oil. Water and solids are sent to the tailings ponds while the remaining solid fines and froth are sent to the froth treatment plant. The froth is mixed with solvents to lower the bitumen viscosity and to allow the final separation of the bitumen from the water and solids. The water extraction process understandably uses a large amount of water. The water usage varies from 5 to 9 m3 of water per m3 bitumen depending on the quality of the ore. A significant portion of this water is recycled back from the tailings ponds where the solids have settled out. Nevertheless, about 3 to 5 m3 of water per m3 bitumen is made up from fresh water sources. Energy use varies from 3 to 5 GJ/m3 of bitumen; much of this energy is spent on heating and moving water about. A significant amount of heat is lost when the hot water is sent to the tailings ponds. This loss varies from 1 to 2 GJ/m3 of bitumen with an average of 1.7 GJ/m3. An important disadvantage of this system is that the fines, which are concentrated in the interstitial water within the ore, are mixed with large volumes of water, diluting them. The interstitial water/fines mixture is a difficult separation by-product to re-concentrate and to dispose of Fines do not settle out of the water easily, even after decades. The mixed fines and water build up over time, requiring ever larger settling ponds, called tailings ponds, to be constructed. These ponds have attracted much adverse attention because flocks of migrating wildfowl have been trapped and killed in the oily tailings. The government of Alberta has directed that the current system is unsustainable. The current tailings ponds must be cleaned up and future facilities must include some, as yet undefined, improved technology to reduce pond size or eliminate them entirely.
Solvent Based or Solvent Assisted Mining Extraction
Numerous patents disclose the use of solvents for bitumen extraction in mining applications. Most of these utilize a single heavy solvent that remains as a liquid at atmospheric pressure and temperature in a continuous fashion. The continuous nature of these methods and machines require that the oil sand and solvent be mixed at atmospheric pressure. These methods and machines do well in separating the bitumen from the oil sand but have difficulty with the fines that are carried away with the solvent bitumen mixture. Because the solvent-bitumen mixture still has a relatively high viscosity, the fines are not easily separated. If lighter solvents are used, they tend to evaporate and are lost to the atmosphere.
A number of patents overcome this disadvantage by using a two solvent system. The first solvent is a heavy solvent, such as naphtha, which acts to separate the bitumen from the coarse sand and also as a slurrying agent to allow the mixture to be transported to pressurized containers. The second solvent, such as propane, butane or hexane, is used to wash the first solvent, lowering its viscosity and allowing complete separation of the fines. Recent examples of such patents include Canadian patent 2,582,078 by Willem Duyvesteyn, et al., Canadian patent application 2,724,806 by Olusola Adeyinka, et al., and Canadian patent 2,520,943 by Vining Wolff, et al.
U.S. Pat. No. 7,384,557 (the '557 patent) describes the use of a single solvent, including paraffinic solvents with both batch and continuous embodiments. This technique uses a series of screws or solids piston pumps to move the ore to several pressurized extraction chambers. The batch embodiment utilizes a complex system and involves multiple filters and a liquid-liquid separation unit.
Improvements of the Proposed Method and Machine Over Hot Water Processes
The proposed method and machine constitutes an improvement over the hot water extraction process because it accomplishes the majority of the work done by the hot water separation systems in one vessel. It radically reduces the amount of water and solids being physically moved, heated, stored and recycled. The four main areas of improvement are:
1. Decreased Capital Costs
Less equipment will be required by this proposed invention, and the required equipment will be smaller. The water supply and preheating and storage systems will be much smaller than the current system in capacity. No slurry preparation or hydrotransport or primary extraction systems will be required. They are replaced by the batch separation machines. The froth treatment systems will be replaced by water wash systems of modest size since they are handling concentrated mixtures, not low density froth streams.
2. Lower Heat Energy Use
As noted above, the Clark process uses 3-5 GJ/m3 of bitumen produced, with heat losses to tailings ponds averaging 1.7 GJ/m3. The proposed system is expected to have three main heat loads, as described in Table 3:
TABLE 3Single light Solvent Method Energy UseFIG. 1Solvent recoveryOre heatingWater heatingTotalCurve #GJ/m3 of bitumen10.410.530.281.2220.410.630.361.4030.410.790.441.64
It can be clearly seen that the total heat usage is less than the Clark water heating losses alone.
3. Diminished Water Use
As noted above the Clark process uses 5 to 9 m3 of water per m3 of bitumen produced. The net usage of water by the Clark process is 3 to 5 m3 of water per m3 of bitumen produced. This is compared to a range of 1 to 2 m3 water per of bitumen for this proposed method and machine.
4. Tailings Ponds Reduced or Eliminated
The quantity of water being used in the applicant's method to separate the fines from the solvent-bitumen mixture for average and high grade ore (FIG. 1, curves 1, 2 and 3) is such that it can be returned to the mine pit without creating any tailings ponds. Low grade ores with high fines contents (>25% by wt %) may require tailings ponds. It is expected that these ponds would be very much smaller than those currently required.
Improvements Over Solvent Extraction Methods
Solvents which are liquids at atmospheric pressure and moderate temperatures have viscosities that are too high to permit efficient separation of fines from the solvent-bitumen mixture. Therefore, these prior art patents are not economically viable unless large settling tanks are used to hold the solvent-bitumen mixture for long periods of time to allow settling, or large numbers of centrifuges are used to accelerate the separation. The applicant's proposed method and machine constitute an improvement over single heavy solvent methods and machines because of its ability to separate a large proportion of the fines in the machine itself, leaving the remainder in a low viscosity fluid that allows further separation with relative ease. The recovery of light paraffin solvents requires much less energy use than that for heavy solvents that are conventionally used.
The applicant's method and machine constitute an improvement over dual solvent methods because the same result is accomplished with substantially less equipment, since only a single solvent recovery system is required. The light solvent is used to maximum advantage in the first separation, causing more than 80% of the solids to be removed in that first step. It is expected to use less energy since only one solvent needs to be recovered and because the recovery of light paraffin solvents requires less heat. Such light solvents flash to vapor using the heat stored in the bitumen, and they may immediately be recovered by condensation.
Improvements Over the Single Paraffinic Solvent System
The system described in U.S. Pat. No. 7,384,557 has a number of characteristics that are not required or are opposite to the applicant's proposed method and machine, as described below.    i) augers and paddles are required to move and mix the material.
The applicant's method requires no mechanical machinery for moving or mixing solids.    ii) a system of inert gas injection is required to maintain pressure. This is required because the temperature must be kept as low as possible to avoid denaturing of the oil, or the process will not work properly.
The applicant's proposed method teaches away from this '557 disclosure and in fact works better at higher temperatures. In the applicant's method, the pressure is maintained by having sufficient liquid solvent in the container so that the pressure will be the vapor pressure of the solvent at the bulk temperature of the materials in the container.    iii) a solvent injection system which enters the container in a manner to cause a vortex flow.
The circulation of solvent in the applicant's method is upwards rather than radial. In this manner, small solid lumps are carried upwards, larger lumps fall and abrade against each other. Further, the turbulent mixing at the bottom of the container causes more abrasion and mixing.    iv) the batch method described in the '557 patent is based on a filter separation method. Application of filters to oil sands is problematic. The filter in the container must be physically robust to withstand battering when the ore is dumped in from above, yet the filter passing size must be quite small, around 0.1 mm (100 μm) to retain even half of the solids in the container. The filter would be subject to clogging with fines and would be difficult to clean.
The applicant's proposed design is based on a gravity separation method. The use of gravity settling is a substantial and non-obvious change from the conventional filter design.